How desirable is living in New York? So desirable that 8.2 million of us cram into a mere 301 square miles, creating astronomical rents (and real-estate prices), just to be a part of it. No wonder, like it or not, this is a roommate city. And these days, New Yorkers are ­pairing up—even tripling, quadrupling, and 28-upling—more than ever before. Census data shows the number of roommates living together in nonfamily households increased more than 40 percent between 2000 and 2010. Here, a glimpse into the roommating universe and how to navigate it, including a foolproof formula for fairly dividing the rent and, for those of you currently on the hunt for a new roomie, insight from a psychopath expert on how to avoid living with one.